This invention relates to a pedal-actuated braking pressure generator for a brake system with anti-lock control comprising a vacuum-operated servo unit which provides both boosting of the braking force and the generating of an auxiliary energy opposed to the pedal force, and of a master brake cylinder connected downstream thereof.
Known braking pressure generators such as German Published Patent Applications (DE-OS No. 33 17 629 and DE-OS No. 34 28 869) use an available vacuum source as a source for the energy required for anti-lock control and for the boosting of the braking force. The servo-unit of these known braking pressure generators is similar to known vacuum brake boosters to a large extent with the specific feature, however, that, by means of the temporary reversal of the pressure conditions in the two working chambers of the servo unit, energy is generated in opposition to the pedal force. In this manner, as soon as any wheel tends to lock because of excessive braking force, a reduction of the braking pressure results and, thereby, a re-stabilization of the course of the wheel. Since in such a device the braking pressure is automatically changed in a like manner in all circuits connected to the master brake cylinder, wheel valves are inserted into the pressure medium paths leading from the master brake cylinder to the wheel brakes. The wheel valves are changed over to lock independently of one another. In accordance with a so-called multiplexing method, namely a consecutively operated method, it is possible to adjust the braking pressure by means of the valves in a known manner individually for each wheel. The vacuum for boosting the braking force and for braking pressure modulation is usually supplied by the suction system of the driving engine of the vehicle. It is possible to manufacture such a brake system at reasonable cost because of the absence of a separate auxiliary energy source and due to the limitation to only three or four wheel valves in all, as a consequence of the multiplex operation.
A disadvantage in brake systems of this type is that during anti-lock control, the auxiliary energy which is opposed to the pedal force retroacts on the brake pedal which is awkward for and may be irritating to the driver. It is thus an object of the present invention to lock the pedal in its position by means of convenient measures upon the onset of anti-lock control and thus to avoid any irritation to the driver which might lead to erroneous reactions. It should be noted that anti-lock control normally comes about or is operative during panicky braking operations and in other dangerous situations.